This invention relates generally to ceramic articles. More specifically, it relates to methods for successfully drying ceramic articles which have been prepared by techniques which introduce liquids into the articles, e.g., gel-casting techniques.
Ceramic articles are often prepared by techniques which involve the use of liquids, such as water. As an example, an article can be prepared from a ceramic slurry which contains the ceramic component itself (e.g., alumina), along with an aqueous or aqueous/organic solution of a polymerizable binder. The slurry is formed into a green product of a desired shape, and then heated. The heating step vaporizes the liquid in the solution, and polymerizes the binder. Continued heating (typically at temperatures of about 300-1350xc2x0 C.) vaporizes or xe2x80x9cburns offxe2x80x9d the binder itself. The green product is then sintered into a final article.
One particular process of this type is referred to as xe2x80x9cceramic gel-castingxe2x80x9d. In such a process, a ceramic slurry containing a solvent and one or more appropriate monomers is poured into a tool. The slurry is then xe2x80x9cgelledxe2x80x9d through a catalyzed reaction which polymerizes the monomer, resulting in a rigid solid (sometimes referred to as a polymer-solvent gel matrix). The polymerized component and solvent are easily removed by heating. Thus, the gel-casting of ceramic materials is a very useful process for fabricating complex shapes, e.g., tubing, rings, pipes, sleeves, high-temperature light fixtures, and various turbine engine components. Such materials often have critical advantages in many applications. For example, they can withstand much higher temperatures than molded plastics, and are much more resistant to corrosion than many metals.
While the removal of liquids from green ceramic articles can be carried out quickly, serious problems can sometimes occur. Drying procedures can result in shrinkage and warpage of the article, as capillary forces draw the ceramic particles together. Green parts containing high levels of liquids often exhibit the most shrinkage. Moreover, parts which include both thin cross-sections and thicker cross-sections are very susceptible to cracking or distortion, as the thin sections dry faster than the thicker sections.
Various techniques have been used to minimize these problems. For example, drying the part very slowly in a relative humidity chamber for a period of time can slow down the evaporation rate. Migration of the liquid in the part from interior regions to the surface occurs. As drying progresses, the temperature of the part can be gradually raised while the humidity is gradually lowered, so that evaporation and migration rates remain somewhat reasonable. As another aid, the part can be rested on special supports during the drying process, to minimize distortion.
Nevertheless, additional advances in removing volatile liquids from green ceramic parts are necessary. Slowing down the drying times does not always prevent shrinkage or warpage. Furthermore, increased drying times may greatly decrease the overall efficiency of the ceramic fabrication process.
The problem is especially severe in the case of a part which requires precise dimensions, and which may include many different cross-sectional dimensions throughout its volume. As an example, ceramic cores which are used to define hollow regions in metal castings are often made by the slurry process. The cores must be manufactured to dimensional precision, corresponding to the dimensions of the desired metal casting. Improved techniques associated with making such cores (as well as other slurry-derived components) would be welcome in the art.
A primary embodiment of this invention is directed to a method of removing substantially all of the volatile component in a green, volatile-containing ceramic article. The method comprises these steps:
(a) freezing the ceramic article; and
(b) subjecting the frozen article to a vacuum for a sufficient time to freeze-dry the article.
The volatile component in the ceramic article often (but not always) includes water. As described below, the type of individual volatile component depends in part on how the ceramic article is prepared. For example, an article formed from a gel-casting slurry may include water, an alcohol such as tert-butyl alcohol, and the ceramic components, e.g., alumina and aluminum.
Freeze-drying of the article usually takes place in a freeze-dryer, under as low a vacuum as possible. In preferred embodiments, the article is heated while being freeze-dried, up to a temperature of at least about 20xc2x0 C.
The ceramic article can be formed from a variety of techniques, such as extrusion, injection molding, or casting. As described below, gel-casting is often the preferred technique. While many ceramic articles can be prepared by this method, cores used in investment casting are often of primary interest. Thus, another embodiment of this invention relates to a method for fabricating a ceramic article suitable for use as a core in the investment casting of directionally solidified and/or single crystal superalloy materials.
As described in the disclosure which follows, use of this invention to remove the volatile component in ceramic articles results in products which do not exhibit any significant warpage upon drying. Moreover, the articles exhibit improved shrinkage-reproducibility. These attributes are especially important when components which require precise dimensions are being formed. Moreover, the relative speed of the process in drying a component can greatly enhance the efficiency of a ceramic fabrication process.